1. Field of the Invention
The invention relates to utilization of communication resources in cellular communication systems and in particular, but not exclusively, to supporting broadcast communication in a time-division duplex 3rd Generation Partnership Project (3GPP) wireless communication system.
2. Description of Related Art
Currently, 3rd generation cellular communication systems are being developed to enhance the communication services provided to mobile wireless users. Some widely adopted 3rd generation communication systems are based on Code Division Multiple Access (CDMA) and Frequency Division Duplex (FDD) or Time Division Duplex (TDD) technology. In CDMA-only systems, user separation may be obtained by allocating different spreading and/or scrambling codes to different users on the same carrier frequency and in the same time intervals. This is in contrast to time division multiple access (TDMA) systems, where user separation is achieved by assigning different time slots to different users.
In addition, TDD provides for the same carrier frequency to be used for both uplink transmissions, i.e., transmissions from the mobile wireless communication unit (often referred to as wireless subscriber communication unit or user equipment, “UE”) to the communication infrastructure via a wireless base station serving a cell, and downlink transmissions, i.e., transmissions from the communication infrastructure to the mobile wireless communication unit via a wireless base station. In TDD, the carrier frequency is subdivided in the time domain into a series of timeslots. The single carrier frequency is assigned to uplink transmissions during some timeslots and to downlink transmissions during other timeslots. An example of a communication system using this principle is the TDD option of the Universal Mobile Telecommunication System (UMTS), as specified in part by 3GPP.
In a conventional cellular system, base stations serving coverage cells in close proximity to each other are allocated non-overlapping transmission resources. For example, in a CDMA network, base stations serving cells within close proximity to each other can be allocated distinct spreading codes (to be used in both the uplink direction and the downlink direction). This may be achieved by, for example, using a common spreading code for both uplink and downlink at each cell, but a different cell-specific scrambling code for each UE within the cell. The combination of these leads to effectively distinct spreading codes at each cell.
In order to provide enhanced communication services, 3rd generation cellular communication systems are designed to support a variety of different and enhanced services. One such enhanced service is multimedia services. The demand for multimedia services that can be received via mobile phones and other portable devices is expected to grow rapidly over the next few years. Multimedia services, due to the nature of the data content that is to be communicated, generally require high bandwidth transmission channels.
Typically, in such cellular systems that employ a single carrier frequency, a wireless subscriber unit is ‘connected’ to one wireless serving communication unit, i.e., a base station of one cell. Base stations in other cells of the network typically generate interfering signals to the wireless subscriber unit of interest. These interfering signals can degrade the maximum achievable data rate that can be maintained with the wireless subscriber unit.
One approach for the provision of multimedia services is to ‘broadcast’ the multimedia signals, as opposed to sending the multimedia signals in a unicast (i.e., point-to-point) manner. Typically, tens of channels carrying, for example, news, movies, sports, etc. may be broadcast simultaneously over a communication network.
As radio spectrum is at a premium, spectrally efficient transmission techniques are required in order to provide users with as many broadcast services as possible, thereby providing mobile phone users (subscribers) with the widest choice of services. It is known that broadcast services may be carried over cellular networks, in a similar manner to conventional terrestrial television/radio transmissions.
Technologies for delivering multimedia broadcast services over cellular systems, such as the Mobile Broadcast and Multicast Service (MBMS) for UMTS, have been developed over the past few years. In these broadcast cellular systems, the same broadcast signal is transmitted over non-overlapping physical resources on adjacent cells within a conventional cellular system. Consequently, at the wireless subscriber unit, the receiver must be able to detect the broadcast signal from the base station of the cell to which it is connected. Notably, this detection needs to be made in the presence of additional, potentially interfering broadcast signals, transmitted on the non-overlapping physical resources of adjacent cells.
In addition, digital video broadcasting (DVB) technologies have recently evolved and are targeted at delivering broadcast video to mobile handheld (DVB-H) terminals. Often, wireless infrastructure transmitters in such networks operate as wireless repeaters. Hence, a separate and distinct technology, usually a cellular phone technology, is used to provide uplink and downlink unicast signals (which are required to carry control signaling and uplink user traffic) to facilitate broadcast communications to the DVB-H terminal using DVB. In this scenario two independent networks are required (cellular and broadcast networks) which tend to operate at substantially different carrier frequencies. This can have implications from an infrastructure perspective in terms of cost and cell site equipment reuse.
Proposed or implemented techniques for broadcast wireless transmissions require either separate spectrum dedicated for broadcast purposes, or duplicate circuitry in the mobile receiver to receive distinct broadcast and unicast transmissions at respective frequencies.
Thus, typically in a wireless communication network, in order to achieve the high bandwidths envisaged for broadcast transmissions, interference from neighboring cells should be mitigated to achieve the high throughput rates required for a broadcast transmission. Hence, an improved mechanism to address the problem of supporting broadcast transmissions over a cellular network would be advantageous. In particular, a system allowing for the provision of broadcast transmissions in a cellular system to co-exist with the existing cellular system would be advantageous.
In one approach, a cellular network is used to deliver downlink broadcast services in addition to unicast services in both the uplink and the downlink modes such that broadcast services are transmitted simultaneously using identical physical resources by either base stations in all cells of the network or by base stations in a cluster of cells in proximity to each other while unicast services are delivered over non-overlapping physical resources in adjacent cells, as in conventional cellular systems.
For example, in a TD-CDMA based cellular system, the broadcast services can be transmitted over the entire network or over a cluster of cells using the same spreading code(s) while unicast traffic is transmitted using distinct spreading codes in adjacent cells. Furthermore broadcast and unicast transmissions may be performed in different timeslots. This reduces the interference experienced by the mobile receiver while detecting broadcast signals. A requirement is to ensure that the signals from the different cells are quasi-synchronous, meaning that signals from different base stations arrive at a UE receiver within a nominal time window.
The data packets arriving at the base stations from the network controllers must arrive at substantially the same point in time so as to be transmitted at the same point in time. It is assumed that the base stations in the network are synchronized via a synchronization port. Typically GPS is one method for the synchronization of base stations in 3GPP UMTS. The GPS signal is applied to a synchronization port of the base station.
Typically the network controllers are asynchronous. In such a case, the data packets may arrive at the base stations at substantially different points in time. Therefore, even though the base stations are synchronized locally, and synchronized at the air-interface, the transmissions from cells when observed at a UE receiver may appear to be asynchronous at the data packet level. If they do appear out-of-sync at the packet data level, then the signals from the out-of-sync cells will appear as interference because different data packets are simultaneously being transmitted by different base stations. Consequently, the signal-to-noise ratio can degrade and hence the throughput may decrease. Therefore, data packets need to be synchronized at the base stations as well as the air-interface.